1. Field of the Invention
The present invention relates to a liquid crystal display device that divides a liquid crystal layer that is vertically aligned in multiple domains.
2. Description of the Related Art
Presently, a transmission type liquid crystal display device of a wide viewing angle such as an IPS (In Plane Switching) type or a VA (Vertical Alignment) type has been spread as a monitor, has improved the response characteristic, and has been used as a television. The liquid crystal display device for the television has been increasingly enlarged in the screen, and there is a case in which plural users watch the large-screen liquid crystal display device at the same time. Under the circumstances, it is desirable to provide the wide viewing angle display that is capable of obtaining substantially the same viewing performance as that in a normal direction even if the users watch the display from an oblique direction.
On the other hand, the liquid crystal display device has been also spread in a portable device including a cellular phone and a digital camera. The portable device is mainly privately used. In recent years, the liquid crystal display device having a display portion that is variable in the angle is increased. As a result, because there are many cases in which the user watches the liquid crystal display device from an oblique angle with respect to the normal direction of the display surface, the wide viewing angle display is desired.
Also, because the display device for the portable information device is used under the various environments from the outdoors in fine weather to the dark room, the transflective display device is desired. The transflective display device includes a reflective portion and a transmissive portion within one pixel. The reflective portion reflects a light that is inputted from the surroundings with the use of a reflector and conducts display. Therefore, because the contrast ratio is constant regardless of the brightness of the surroundings, the excellent display is obtained under the relative bright environments from the outdoors in fine weather to the dark room. The transmissive portion uses a back light, and the brightness is constant regardless of the surroundings. Therefore, the display of the high contrast ratio is displayed under the relative dark environments from the indoors to the dark room. The transflective type liquid crystal display device having both of the reflective portion and the transmissive portion is capable of obtaining the display of the high contrast ratio under the wide range environments from the outdoors in fine weather to the dark room.
The VA type liquid crystal display device is characterized in that retardation is 0 nm since a liquid crystal layer is vertically aligned at the time of applying no voltage, and the display of the high contrast ratio is relatively easily obtained when polarizers are disposed on the upper and lower surfaces of the liquid crystal layer in a cross nicol arrangement. In order to make the transflective VA type liquid crystal display device, a quarter wave plate or a laminated body consisting of a half wave plate and a quarter wave plate is disposed between each of the polarizers and the liquid crystal panel. In this case, the slow axis directions of the upper and lower quarter wave plates, or the upper and lower laminated bodies each consisting of the half wave plate and the quarter wave plate are made perpendicular to each other, to thereby relatively readily obtain the display of the high contrast ratio.
The VA type liquid crystal display device has suffered from such a problem that the gray scale reproductivity is deteriorated in the oblique direction. For example, in the case where an image having the gray scale such as a photograph is displayed and watched from the oblique direction, the brightness of the halftone is shifted to a higher tone side, and the image looks whitish. Also, in the VA type liquid crystal display device of the transflective type, since black display is also shifted to the higher tone side in addition to the halftone, the contrast ratio is deteriorated in addition to the excessively brightened image.
In the liquid crystal display device, display is conducted by using a change in the alignment state of the liquid crystal layer with application of an electric field, and a change in the optical characteristic with the change in the alignment state. The change in the alignment state of the liquid crystal layer is classified into two types consisting of a change in the rotation within a liquid crystal layer plane, and a change in a slope angle with respect to a direction perpendicular to the liquid crystal layer plane.
In the IPS type liquid crystal display system, the main change is the change in the rotation within the liquid crystal layer plane, and the change in the slope angle with respect to the direction perpendicular to the liquid crystal layer plane is small. On the contrary, in the VA type liquid crystal display device, the main change is the change in the slope angle of the liquid crystal layer, and the rotation within the liquid crystal layer plane is small.
Presently, the change in the alignment state of the VA type liquid crystal display device is similar to an ECB (Electrically Controlled Birefringence) type liquid crystal display device that is used in the transflective type liquid crystal display device. In the ECB type liquid crystal display device, because the slope direction of the liquid crystal layer at the time of applying the voltage is uniform, a liquid crystal polymer film whose single alignment direction is inclined is laminated on the ECB type liquid crystal display device in such a manner that the monocular alignment direction becomes perpendicular to the slope direction of the liquid crystal layer. As a result, the tone shift in the oblique direction can be compensated to some degree. On the other hand, the ECB type liquid crystal display device suffers from such a problem that the azimuth dependency of the viewing performance is large. This is caused by inclining the liquid crystal layer over the entire pixels in one direction at the time of applying a voltage as described above.
On the contrary, the VA type liquid crystal display device is characterized in that plural minute areas (domains) which are different in the liquid crystal alignment direction from each other at the time of applying the voltage are produced within the pixel, and this structure is called “multi-domain structure”. The oblique characteristics of the individual domains are fundamentally equal to those of the liquid crystal layer of the ECB type liquid crystal display device, and are compensate between the respective domains, to thereby eliminate the azimuth dependency of the oblique characteristic. However, since the slope direction of the liquid crystal layer is not uniform, the tone shift eliminating method of the oblique direction using the liquid crystal polymer film such as the ECB type liquid crystal display device is not effected.
There has been known that it is preferable to give a distribution to the slope angle in order to reduce the toner shift in the oblique direction of the VA type liquid crystal display device. As the method of giving the distribution to the slope angle, there is, for example, a method of arranging two areas that are different in the transmittance-to-applied voltage characteristic within the pixel.
More particularly, a dielectric layer is partially disposed between the liquid crystal layer and the electrode, and a voltage value that is applied to the liquid crystal layer is partially changed within one pixel. Alternatively, two kinds of active switches which are different in the threshold voltage, and a pair of electrodes that are connected to those active switches, individually, constitute one pixel. Also, the same effect is obtained when two retention capacitors which are different in the capacitance from each other, and a pair of electrodes that is connected to the respective retention capacitors, individually, constitute one pixel.
However, since those methods are complicated in the pixel structure and insufficient in the distribution of the slope angle within one pixel, the tone shift in the oblique direction of the VA type liquid crystal display device is not sufficiently reduced.
Japanese Patent Laid-Open No. 2004-102001 discloses a liquid crystal display device in which a vertical electric field is applied to a liquid crystal layer with the use of a comb-like electrode that is so shaped as to be widened in a radial fashion from the center of one pixel to drive the liquid crystal display device. A pixel electrode and a common electrode are disposed in proximity to each other in the vicinity of the center of the radial comb-like electrode structure, and microscopic minimum processing dimensions are required to form the above structure. For example, in the case of the microscopic pixel corresponding to the high resolution display, there is a case in which the number of comb-like electrodes that can be arranged within one pixel is reduced, and the radial comb-like electrode structure cannot be formed. Also, the radial comb-like electrode structure may not be formed because the microscopic structure that is equal to or lower than the minimum machining dimensions is required in the vicinity of the center. In this case, the aperture ratio is deteriorated in the center of the pixel because no voltage can be applied to the liquid crystal layer, and it is predicted that the transmittance is deteriorated.
In the present invention, a voltage is applied to a liquid crystal layer that are vertically aligned with the use of a comb-like electrode of the IPS system. In the VA type liquid crystal liquid crystal display device, the voltage is applied to the liquid crystal layer with the use of the pair of electrodes that face each other through the liquid crystal layer. In this case, the electric field that is applied to the liquid crystal layer is substantially uniform over the entire pixel.
On the contrary, the electric field that is developed by the comb-like electrode is not uniform. When attention is paid to the respective portions viewed cross the comb-like electrode, for example, the center of the comb-like electrode, the center of the gaps of the comb-like electrode, and the end of the comb-like electrode, the electric field is different in the intensity and direction at the respective portions. Therefore, the alignment direction and the slope angle of the liquid crystal layer are different at the respective portions that cross the comb-like electrode.
The electric field direction is close to the normal direction at the end of the comb-like electrode. When the dielectric anisotropy of the liquid crystal layer is positive, the liquid crystal layer tries to be aligned in the direction of the line of electric force, a change in the alignment of the liquid crystal layer hardly occurs at the time of applying the voltage.
Also, when the dielectric anisotropy of the liquid crystal layer is positive, there are many cases in which the liquid crystal layer is going to change the alignment in the opposite direction at both ends of the comb-like electrode. The electric field does not exist in the center of the comb-like electrode even at the time of applying the voltage, and the liquid crystal layer in the center of the comb-like electrode changes the alignment in accordance with the elastic deformation that propagates from the liquid crystal layer in the vicinity of the comb electrode center. In this way, when the dielectric anisotropy of the liquid crystal layer is positive, there are many cases in which the elastic deformation is opposite in the direction to each other. In this case, the elastic deformation is balanced with each other in the center of the comb-like electrode, and no change in the alignment occurs.
On the contrary, because a sufficient change in the alignment occurs even at the end or the center of the comb-like electrode, the liquid crystal layer is made of a liquid crystal mixture that is negative in the dielectric anisotropy. In the case where the dielectric anisotropy is negative, the liquid crystal layer tries to be aligned in a direction perpendicular to the line of electric force, and because the degree of freedom of alignment is high, a change in the alignment is liable to occur. In this way, although the alignment state of the liquid crystal layer is different at the respective portions of the comb-like electrode, because the distribution range of the azimuth in the liquid crystal alignment direction is limited, the multi-domain structure is required even in the combination of the vertically aligned liquid crystal layer with the comb-like electrode.
First of all, when it is assumed that the change in the alignment of the liquid crystal layer is limited to in-plane that crosses the comb-like electrode, in the case where the interval of the comb-like electrodes is sufficiently narrow, a spray deformation of the liquid crystal layer becomes large at the time of applying the voltage. In order to alleviate the spray deformation of the liquid crystal layer, it is desirable that the alignment is changed in a direction perpendicular to a plane that crosses the comb-like electrode, that is, in a direction parallel to the comb-like electrode. However, there exist two directions that are in parallel to the comb-like electrode. When a change in the alignment in any one of those directions is made easier, a change in the alignment in a direction parallel to the comb-like electrode is liable to occur. That is, when two or more microscopic areas that are different from each other in a direction along which the change in the alignment is easy are formed within one pixel, the multi-domains can be conducted.
As a method of realizing the multi-domains, for example, there is a method in which a projection structure is arranged on the comb-like electrode or on its opposed electrode. The liquid crystal layer tries to be aligned to be perpendicular to the oblique surface of the projection in the vicinity of the projection structure. In addition, when the projection structure is shaped like a strip and distributed substantially perpendicularly to the comb teeth direction, the liquid crystal layer alignment that is in proximity to the projection structure is sloped in any one of directions that are in parallel to the comb-like electrode in a state where no voltage is applied. At the time of applying a voltage, the liquid crystal layer is liable to be sloped in a direction given by the projection structure, and the slope of the oblique surface is reversed with the center of the projection structure as the boundary. As a result, the slope direction of the liquid crystal layer is opposite to each other.
Also, the pitches of the comb-like electrode are continuously changed. In this case, one of the two directions which are in parallel to the comb-like electrode is reduced in the pitch, and the other direction is increased in the pitch. The line of electric force that is developed in the liquid crystal layer and the density of the line of electric force are changed according to the pitches of the comb-like electrode. The resultant change in the alignment of the liquid crystal layer is also changed according to the pitches of the comb-like electrode. Since two directions are not equal to each other, the slope in any one of those directions is eased, and the slope direction is determined. In other words, the comb-like electrodes within one pixel are divided into plural portions that are different from each other in a direction of increasing the pitches, thereby making it possible to form plural portions that are different from each other in the slope direction at the time of applying the voltage within one pixel.
In addition, in the case of the electrode structure in which the comb-like electrodes are concentrated to one point such as the radial structure, the common electrode and the pixel electrode are not formed on the same plane, but both of those electrodes are formed on planes that are different through the insulation layer. In this case, as compared with a case in which the common electrode and the pixel electrode are formed on the same plane, the density of the lines of electric force increases by double. As a result, even in the case where the number of comb-like electrodes which can be arranged within one pixel which is a microscopic pixel that deals with the high precision display, the implement is easy.
The voltage is applied to the liquid crystal layer that is vertically aligned by means of the comb-like electrode when the dielectric anisotropy is negative, and no voltage is applied with the result that a sufficient distribution occurs in the slope angle of the liquid crystal layer within one pixel. In addition, the projection structure is arranged on the opposed electrode, or the pitches of the comb-like electrode structure are continuously changed, thereby making it possible to provide the multi-domains.
As described above, there is obtained the liquid crystal display device that is excellent in the contrast ratio in the normal direction and the oblique direction, small in the azimuth dependency of the viewing performance, and small in the tone shift without deterioration of the gray scale reproductivity even in the oblique direction.
When the above liquid crystal display device is used for a liquid crystal television as the transmissive liquid crystal display device, there is obtained the display high in the contrast ratio with less excessive brightness and excellent in the gray scale reproductivity, even in the case where a large number of persons watch the liquid crystal television, and some of those persons watch the liquid crystal television from the oblique direction.
Also, in the transflective type liquid crystal display device in which the reflective portion and the transmissive portion are arranged within one pixel, there is obtained display that is high in the contrast ratio, excellent in the gray scale reproductivity, and has the wide viewing angle under the wide range of environments. When the above transflective type liquid crystal display device is used for a monitor of a cellular phone, a portable television, a digital steel camera, or a digital video camera, there is obtained display that is excellent in the visibility under the diverse environments from the outdoors in fine weather to the dark room.